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Anyonic excitations emerging from a Kitaev spin liquid can form a basis for quantum computers 1, 2 . Searching for such excitations motivated intense research on the honeycomb iridate materials 3-17 . However, access to a spin liquid ground state has been hindered by magnetic ordering 5 . Cu 2 IrO 3 is a new honeycomb iridate without thermodynamic signatures of a long-range order 18 . Here, we use muon spin relaxation to uncover the magnetic ground state of Cu 2 IrO 3 . We find a two-component depolarization with slow and fast relaxation rates corresponding to distinct regions with dynamic and static magnetism, respectively. X-ray absorption spectroscopy and first principles calculations identify a mixed copper valence as the origin of this behavior. Our results suggest that a minority of Cu 2+ ions nucleate regions of static magnetism whereas the majority of Cu + /Ir 4+ on the honeycomb lattice give rise to a Kitaev spin liquid.

Kitaev magnets are materials with bond-dependent Ising interactions between localized spins on a honeycomb lattice. Such interactions could lead to a quantum spin-liquid (QSL) ground state at zero temperature. Recent theoretical studies suggest two potential signatures of a QSL at finite temperatures, namely a scaling behavior of thermodynamic quantities in the presence of quenched disorder, and a two-step release of the magnetic entropy. Here, we present both signatures in Ag3LiIr2O6 which is synthesized from α-Li2IrO3 by replacing the inter-layer Li atoms with Ag atoms. In addition, the DC susceptibility data confirm absence of a long-range order, and the AC susceptibility data rule out a spin-glass transition. These observations suggest a closer proximity to the QSL in Ag3LiIr2O6 compared to its parent compound α-Li2IrO3 that orders at 15 K. We discuss an enhanced spin-orbit coupling due to a mixing between silver d and oxygen p orbitals as a potential underlying mechanism.

We report on a reoptimization of the Tran-Blaha modified Becke-Johnson (TB-mBJ) potential dedicated to the prediction of the band gaps of 3-dimensional (3D) and layered hybrid organicinorganic perovskites (HOP) within pseudopotential-based density functional theory methods. These materials hold promise for future photovoltaic and optoelectronic applications. We begin by determining a set of parameters for 3D HOP optimized over a large range of materials. Then we consider the case of layered HOP. We design an empirical relationship that facilitates the prediction of band gaps of layered HOP with arbitrary interlayer molecular spacers with a computational cost considerably lower than more advanced methods like hybrid functionals or GW. Our study also shows that substituting interlayer molecular chains of layered HOP with Cs atoms is an appealing and cost-effective route to band gap calculations. Finally, we discuss on the pitfalls and limitations of TB-mBJ for HOP, notably its tendency to overestimate the effective masses due to the narrowing of the band dispersions. We expect our results to extend the use of TB-mBJ for other low-dimensional materials. 2 B. Traore et coll. Phys. Rev.

Great effort has been devoted to developing single-phase magnetoelectric multiferroics, but room-temperature coexistence of large electric polarization and magnetic ordering still remains elusive. Our recent finding shows that such polar magnets can be synthesized in small-tolerance-factor perovskites AFeO 3 with unusually small cations at the A-sites, which are regarded as having a LiNbO 3-type structure (space group R3c). Herein, we experimentally reinforce this finding by preparing a novel room-temperature polar magnet, LiNbO 3-type InFeO 3. This compound is obtained as a metastable quench product from an orthorhombic perovskite phase stabilized at 15 GPa and an elevated temperature. The structure analyses reveal that the polar structure is characterized by displacements of In 3+ (d 10) and Fe 3+ (d 5) ions along the hexagonal c-axis (pseudocubic [111] axis) from their centrosymmetric positions, in contrast to well-known perovskite ferroelectrics (e.g., BaTiO 3 , PbTiO 3 , BiFeO 3) where d 0 transition-metal ions and/or 6s 2 lone-pair cations undergo polar displacements through the so-called second-order Jahn-Teller (SOJT) distortions. Using density functional theory calculations, the electric polarization of LiNbO 3-type InFeO 3 is estimated to be 96 μC/cm 2 along the c-axis, comparable to that of an isostructural and SOJT-active perovskite ferroelectric, BiFeO 3 (90-100 μC/cm 2). Magnetic studies demonstrate weak ferromagnetic behavior at room temperature, as a result of the canted G-type antiferromagnetic ordering of Fe 3+ moments below T N ~ 545 K. The present work shows functional versatility of small-tolerance-factor perovskites and provides a useful guide for the synthesis and design of room-temperature polar magnets.

The synthesis and properties of a novel hetero-tetranuclear compound [Cr(bpy)(μ-O)Nb(CO)]·3HO (1; bpy = 2,2'-bipyridine), investigated by single-crystal X-ray diffraction, magnetization measurements, IR, UV/visible spectroscopy, electron paramagnetic resonance (EPR; X- and Q-bands and high-field), and density functional theory (DFT) calculations, are reported. Crystal structure of 1 (orthorhombic Pcab space group) consists of a square-shaped macrocyclic {Cr(μ-O)Nb} core in which Cr and Nb ions are alternately bridged by oxo ions and three uncoordinated water molecules. The intramolecular Cr···Cr distances through the -O-Nb-O- bridges are 7.410(2) and 7.419(2) Å, while diagonal separation is 5.406(2) Å. The temperature dependence of magnetization M(T) evidences an anti-ferromagnetic ground state, which originates from a magnetic interaction between two Cr ions of spin 3/2 through two triatomic -O-Nb-O- diamagnetic bridges. A spin Hamiltonian appropriate for polynuclear isolated magnetic units was used. The best-fitting curve for this model is obtained with the parameters g = 1.992(3), J = -12.77(5) cm, and |D| = 0.17(4) cm. The Cr···Cr dimer model is confirmed by EPR spectra, which exhibit a pronounced change of their shape around the temperature corresponding to the intradimer coupling J. The EPR spectra simulations and DFT calculations reveal the presence of a single-ion anisotropy that is close to being uniaxial, D = -0.31 cm and E = 0.024 cm.

The present article is a thorough quantum mechanics investigation based on DFT method targeting the opto-electronic properties of the m-ZrO 2 material issuing from the presence of defects. Herein, we conclude that the luminescence observed around 477 nm (∼2.60 eV) corresponds to the charge transfer between Ti Zr and oxygen atoms (i.e., Ti 3+ + O -→ Ti 4+ + O 2 -), and not from oxygen vacancies or d -d transitions at Ti 3+ sites. Namely, based on constrained DFT calculations, an emission at 2.61 eV (475 nm) was calculated that matches perfectly with experiments (around 2.60 eV / 477 nm). Moreover, in order to demonstrate the propensity of the ZrO 2 host lattice to entrap titanium, intrinsic and extrinsic point defect formation energies on m-ZrO 2 were computed.

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